But many smaller retailers’ Facebook stores are prospering, according to a new data from social commerce vendors Ecwid Inc. and Payvment. That might explain why 16% of retailers enable shoppers to buy directly on Facebook, according to the E-Tailing Group’s 14thAnnual Mystery Shopping Study.

Ecwid says that the average revenue of the roughly 35,000 Facebook storefronts it powers increased about 40% in 2011 compared with 2010. Merchants that operate web sites and Facebook storefronts add, on average, 15% to their overall revenue via sales on the social network.

“F-commerce is not only alive and well, it is growing,” says Ruslan Fazlyev, CEO and founder of Ecwid, using the marketing term for Facebook commerce. “While some big brands have stumbled, our data shows the [small- and medium-sized business] market is making progress on Facebook and other social sites.”

The challenge, for small- and medium-sized businesses, is getting shoppers to find their stores on the social network. That’s why a Payvment survey of 750 sellers using its platforms found that 84% of merchants actively market their Facebook presence and 39% do so via Facebook’s various ad formats.

Those Facebook ads are extremely effective, which is why 70% of merchants that have used them in the past plan to do so again, says the survey.

Other ways merchants attract shoppers to their stores on the social network include promoting the presence on their web site (38%), Twitter (34%), e-mail marketing (30%), print ads (12%), direct mail (9%) and Google search ads (8%).

Many of the retailers surveyed say they plan to increase their marketing efforts in the next six months. 28% expect to boost their use of Google ads, 30% plan to run print ads, 29% aim to expand their e-mail marketing programming and 21% plan to promote their Facebook stores on their company web sites.

Most of the merchants using the Payvment platform are small businesses with 72% having less than 500 fans on the social network.

GameStop is No. 113 in the Internet Retailer Top 500 Guide. J.C. Penney is No. 20 and Gap is No. 24.